1. Field of the Invention
The present invention is directed to an RF power amplifier architecture that significantly reduces an amplifier's out-of-band emissions.
2. Background Art
A principle goal of communication systems is to maximize spectrum efficiency via the use of broadband waveforms transmitting over non-contiguous spectrum and to minimize the waveform's adjacent power level. A goal of the Defense Advanced Research Projects Agency (DARPA) in Future Combat Systems (FCS) and other programs is the development of the orthogonal frequency-division modulation (OFDM) for use with tactical systems. The OFDM waveform has perhaps the best combination of multipath and Eb/No properties of any waveform. Unfortunately, the OFDM waveform has a high peak to average voltage ratio, which requires very high amplifier linearity to suppress out-of-band emissions. This is a very significant problem that prevents the OFDM waveform in some scenarios because high power, linear amplifiers are not cost effective in many designs. Some designers believe that the OFDM out-of-band emission problem is so severe that single carrier waveforms with special equalization are the preferred solution (see Falconer, David et al., Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems, IEEE Communications Magazine, April 2002). The proposed technology directly mitigates the OFDM out-of-band emission problem, thus enabling OFDM to be widely applicable in tactical situations.
Minimizing out-of-band and spurious emissions is a very challenging aspect of multi-band radio design. FCC regulations and interference predictions (detailed below) present typical maximum emission values. To achieve low emissions, a combination of high third order intercept amplifiers and tracking filters are required. The filters must be high-Q bandpass filters with the passband set close to the desired output bandwidth. The filters must have high third order intercept points to avoid contributing to the problem they are fixing. These amplifiers and filters are expensive, are large, require high prime power, and are heavy. State-of-the-art broadband, high performance tunable filters are manufactured by PoleZero Corporation. The high IP3 performance (>+50 dBm) PoleZero product is the “Power-Pole” Filter. This device requires approximately 7.5 W of prime power, is several inches in size, and is expensive (˜$2 k each). The frequency coverage is only 10 MHz to 700 MHz and each device has a 3:1 tuning ratio.
The required amplifier performance level to obtain low spurious emissions is difficult if not impossible to easily attain in prior art radios. Many radios economize on this part of the design and suffer serious operational limitations or have great difficulties in getting spectrum authorization.
These problems are compounded in the next generation of multi-band radios because the large frequency range and the use of variable transmit bandwidths increases the number of required filters. The goal of transmitting a non-contiguous spectrum requires even greater filter flexibility to accommodate “tailoring” of the transmitted spectrum. It is believed that a “brute-force” amplifier/filtering approach to achieve acceptably low spurious emissions with a multi-band radio, and to achieve reasonable cost, size, weight, and power goals is not possible with current technology.
The maximum permitted spurious transmitted power levels are quite small and require high RF performance to achieve. The maximum power can be determined via two methods. Both methods are used in the current debate on authorizing Ultra-Wide Band (UWB) devices (see filed comments on Docket 98-153 at www.fcc.gov). This debate is relevant because broadband waveforms create wide bandwidth noise over the approximately the same frequency band that is widely used for terrestrial, tactical communications (20 MHz to 3,000 MHz). Many users within this band have filed comments regarding wideband noise interference applicable to their specific systems, making available detailed, applicable interference analysis.
The first method (“Noise Floor Method”) to estimate the maximum spurious transmitter power level determines the maximum broadband noise level that can be transmitted that would cause a small (3 dB) rise in the victim receiver's thermal noise floor at a certain distance. Using the free-space range equation, omniantennas and a 6 dB victim noise figure, FIG. 13 shows the maximum transmitted noise level in dBm/MHz versus frequency for ranges of 10 meters, 100 meters, and 1000 meters. In most cases, frequency sharing using broadband waveforms will occur with distant primary users at 1000 m range or greater. An exception is the GPS band, which will be heavily used in close proximity (a few meters) to tactical receivers.
The second method (“Part 15 Method”) to estimate the maximum spurious transmitted power level is to adopt FCC Part 15 (CFR Part 15.209), standards for the amount of unintended power radiated. This standard is a field strength level (100 uV/meter from 30-88 MHz, 150 uV/meter from 88 216 MHz, 200 uV/meter from 216-960 MHz, and 500 uV/meter at greater than 960 MHz) at 3 meters range from the device. FIG. 13 shows this level as “Part 15” transformed back to transmitted power using the free-space range equation and omni-antenna gains. Above 960 MHz, the allowed transmission power is −41.2 dBm/MHz.
These two methods provide different power levels, which is partially the key issue in the UWB debate and also creates uncertainty on what levels should be adopted. Several groups have argued that the Part 15 levels are too high for UWB devices and need to be reduced, especially in the GPS bands (1559 MHz-1610 MHz). The FCC has proposed a 12 dB reduction to the Part 15 limit to a value of −53.2 dBm/MHz, FCC Notice of Proposed Rulemaking FCC 00-163. These arguments are related to the high peak to average power ratio of the UWB signal and the large number of expected UWB devices, and may not be applicable to an advanced broadband waveform. The UWB arguments are also focused on interference with nearby (several meters) devices and not with distant (100s of meters) victim receivers. It is believed that the Part 15 requirements are more than adequate for tactical purposes and that the Noise Floor Method is the most applicable. The exception is within the GPS bands wherein the FCC's recommendation of the maximum emission level of −53.2 dBm/MHz appears to be reasonable. Table 1 summarizes the recommended emission levels detailed above.
TABLE 1Allowable Band EmissionsAllowable AllowableEmissionEmissionLevel in aLevel in a1 MHz61 kHzBandBandwidthBandwidthComment30 MHz to −45 dBm/MHz−57.1 dBm“Noise Floor Method”100 MHz100 MHz to−35 dBm/MHz−47.1 dBm“Noise Floor Method”1,000 MHz1,000 MHz to−35 dBm/MHz−47.1 dBm“Noise Floor Method”3,000 MHz1with 20 dB marginto account for directional antennas1559 MHz-−53.2 dBm/MHz  −65.3 dBmGPS band - Follow FCC1610 MHzrecommendation1Except for GPS band.
What these maximum permitted spurious power levels means is that a non-contiguous waveform must have very low out-of-band emissions to avoid causing interference, and a need exists to have systems that are capable of achieving these low out-of-band emissions.
Present day amplifiers are incapable of achieving these low emissions. In this regard, estimates of the out-band emission levels of RF amplifiers with different third order intercept points (IP3) are shown in FIGS. 14A-C. Shown are typical output spectrums with a 20 MHz transmitted waveform with a 5 MHz spectrum gap, 1 W output, and an amplifier IP3 value of +40 dBm (FIG. 14A), +50 dBm (FIG. 14B), and +60 dBm (FIG. 14C). The “noise” level within the spectrum gap (at 100 MHz) is −20 dBm (IP3=+40 dBm) (FIG. 14A), −40 dBm (IP3=+50 dBm) (FIG. 14B), and −60 dBm (IP3=+60 dBm) (FIG. 14C).
Comparing with the allowable emission levels as shown in Table 1 indicates that with this waveform and power level, an output amplifier with IP3˜+55 dBm is required. An amplifier with IP3>+60 dBm is required to protect the GPS band, which is not feasible within the SWP constraints of tactical radios. Current multi-band amplifiers have IP3 values of about +42 dBm, Stanford Microdevices SGA-9289 (IP3=42.5 dBm). Thus, burdensome post amplifier bandpass filtering will be required to achieve low spurious output levels with this example waveform, which we believe is similar to what is envisioned for future tactical radios.
Commercially available multi-band RF amplifiers have limited dynamic range performance. It is likely that commercial amplifiers small enough for tactical operations will not have high enough IP3 performance to produce a useful spectrum notch.
Table 2 shows the performance of several types of commercially available RF amplifiers. The performance of the recently introduced WJ Communications AH1 amplifier is shown in the first row. The WJ AH1 is small enough to be useful for small, handheld devices. It's IP3 level of +41 dBm would produce power out-of-band emissions similar to what is shown in FIG. 14A. The Mini-Circuits ZHL-42W is a connectorized instrumentation that covers about the frequency range of interest (30 MHz to 3,000 MHz), but also has a low IP3 value. The Mini-Circuits ZHL-5W-1 has higher output power and IP3 value, but doesn't cover the upper part of the desired frequency range. This is probably due to stray capacitance effects in the baluns and transformers used in the design.
An extreme example is the Spectran MCPA 4080, which is a narrowband PCS base station amplifier. It has excellent RF performance over a narrow (1930 MHz to 1990 MHz) frequency range and would support a low out-of-band emission, non-contiguous waveform. But the required power level of 1130 W is prohibitive, even for vehicle applications. Thus, current amplifiers are still lacking in supporting a low out-of-band emission, non-contiguous waveform.
TABLE 2Performance comparison of several amplifier types.FrequencyOutputBiasAmplifierTypeRangePowerPowerIP3EfficiencyWJ CommunicationsMMIC250 MHz to+21 dBm2W41 dBm6%AH1component3000 MHzMini-Circuits -Connectorized10 MHz-+28 dBm12.5W38 dBm5%ZHL-42Winstrumentation4200 MHzamplifierMini-Circuits -Connectorized1 MHz-+37 dBm79W45 dBm6%ZHL-5W-1instrumentation500 MHzamplifierAmplifier ResearchRack mounted800 MHz to+39 dBm150W43 dBm5%5S1G4instrumentation4,200 MHzamplifierSpectran MCPA 4080Rack mounted,1930 MHz to+49 dBm1130W81 dBm7%single band1990 MHzPCS basestationamplifier
As such, there is a need for improved amplifiers that reduce an amplifier's spurious emission to maximize spectrum efficiency. The present invention responds to this need by the development of an improved amplifier system that reduces out-of-band emissions, this eliminating significant interference to existing narrow bandwidth users.